Generally, thermal recording materials having a thermal recording layer based mainly on a colorless or pale-colored basic (electron-donating) leuco dye (hereinafter sometimes to be abbreviated as “dye”) and an electron-accepting developer that develops color by reacting with dye on heating (hereinafter sometimes to be abbreviated as “developer”) are widely used in practice. For recording on the thermal recording materials, a thermal printer with a built-in thermal head, and the like are used. This method of thermal recording is advantageous over other conventional recording methods in practical use, with features such as (a) noiselessness during recording, (b) obviation of the need for development and fixation, (c) freedom from maintenance work, (d) relatively inexpensive instrumentation, (e) compactness, and (f) very vivid colors developing in the images obtained, and is widely used for facsimiles, computer terminal printers, automated ticket machines, measurement recorders, handy terminals for outdoor use, and the like. In recent years, to reduce consumption power by down-sizing these instruments, a material showing high color-developing sensitivity even with low energy, free of background fog (unintended color development on a white blank area due to heating during storage and the like), and showing superior image stability is required.
A factor having a great influence on the color-developing sensitivity is selection of the dye and developer (particularly developer) constituting the thermal recording layer. Various developers have been studied heretofore. For example, benzyl p-hydroxybenzoate expected at one time as a high sensitive developer is no longer used at present, since image stability is markedly low. In addition, bisphenol A (that is, 4,4′-isopropylidenediphenol) shows insufficient color-developing sensitivity and unsatisfactory image stability such as plasticizer resistance and heat resistance. Furthermore, since bisphenol A is suspected to have an environment hormone activity, the world's strong tendency is to ban the use thereof. In fact, some countries have banned the use of a thermal recording material using bisphenol A. To solve this problem, bisphenol S (i.e., 4,4′-dihydroxydiphenylsulfone) is sometimes used as a developer. However, bisphenol S is disadvantageous in that it has high melting point and low color-developing sensitivity.
In addition, patent document 1 describes use of a phenol derivative containing an aryloxysulfonyl group optionally having substituent(s) as an electron accepting compound (developer), and recites a chlorine atom, an alkyl group and the like as the substituent of the aryl group. However, patent document 1 does not describe an alkoxycarbonyl group and a carboxy group as the substituent of the aryl group. While the aryloxysulfonyl group-containing phenol derivative described in patent document 1, which has a chlorine atom or an alkyl group as the substituent of the aryl group, shows good color-developing sensitivity but defectively shows inferior background fog and unclear contrast between blank area and recorded area.